As integrated circuits have become increasingly smaller, electrically conductive structures within the integrated circuits are placed increasingly closer together. This situation tends to enhance the inherent problem of parasitic capacitance between adjacent electrically conductive structures. Thus, new electrically insulating materials have been devised for use between electrically conductive structures, to reduce such capacitance problems. The new electrically insulating materials typically have lower dielectric constants, and thus are generally referred to as low k materials. While low k materials help to resolve the capacitance problems described above, they unfortunately tend to introduce new challenges.
Low k materials are typically filled with small voids that help reduce the material's effective dielectric constant. Thus, there is less of the material itself within a given volume, which tends to reduce the structural strength of the material. The resulting porous and brittle nature of such low k materials presents new challenges in both the fabrication and packaging processes. Unless special precautions are taken, the robustness and reliability of an integrated circuit that is fabricated with low k materials may be reduced from that of an integrated circuit that is fabricated with traditional materials, because low k materials differ from traditional materials in properties such as thermal coefficient of expansion, moisture absorption, adhesion to adjacent layers, mechanical strength, and thermal conductivity.
Concerning the problem of thermal coefficient of expansion, when adjacent layers having different thermal coefficients of expansion are heated and cooled, such as occurs during the normal operation of an integrated circuit, the layers tend to expand and contract at different rates and to different degrees. These forces induce strains in the adjacent and proximal layers. Low k materials tend to have thermal coefficients of expansion that are sufficiently different from those of the other materials used to fabricate integrated circuits, that such strains create problems, which may be both physical and electrical, in the integrated circuit.
As to the problem of moisture absorption, the porosity of low k materials makes them susceptible to absorbing the moisture that typically tends to diffuse into the packaged integrated circuit. As the low k material absorbs such moisture, the properties of the low k material changes. For example, the dielectric constant of the material changes, because the voids tend to fill with water or water vapor. The moisture in the voids may rapidly expand during subsequent heating operations such as baking or solder ball reflow, causing the layers of the integrated circuit to burst apart, resulting in dramatic device failure. The moisture absorbed by the low k material also tends to cause other problems, some of which are described in more detail below.
Integrated circuits containing low k materials are inherently more prone to delamination, either between the various layers of the integrated circuit itself, or between the integrated circuit and packaging materials, such as underfill and mold compound, or other materials which are in intimate contact with the integrated circuit. There are several probable causes for such delamination, including a reduction in the adhesion of a low k layer due to absorption of moisture, as described above. In addition, because the low k material tends to be very porous by nature, there is physically less material available to form adhesive bonds with adjacent layers. Further, the strains induced by differing thermal coefficients of expansion also tend to shear the low k layer from adjacent layers, which again tends to enhance the occurrence of delamination.
As to mechanical strength, low k materials are typically more brittle and have a lower breaking point than other materials. One reason for this is, again, the porosity of the low k material, where a significant percentage of its physical volume is filled with voids. Thus, integrated circuits containing low k materials are inherently more prone to breaking or cracking during processes where physical contact is made with the integrated circuit surface, such as wire bonding and electrical probing, or processes that cause bending stresses such as mold curing, underfill curing, solder ball reflow, or temperature cycling.
Finally, because of their porosity and other properties, low k materials tend to be very poor thermal conductors, typically much less than half a watt per meter-Kelvin (W/mK). This contrasts significantly with the thermal conductivity of traditional integrated circuit and packaging materials such as silicon (60-90 W/mK), copper 380-390 W/mK), mold compound (0.7-2 W/mK), or die attach material (2-4 W/mK). Thus, the thermal energy created during the normal operation of the integrated circuit tends to not be dissipated well by low k materials. Therefore, thermal energy tends to build up within the integrated circuit, and is expressed as localized areas of increased temperature, or hot spots.
There is a need, therefore, for new structures, processes, and materials for use in integrated circuit fabrication, which help to alleviate one or more of the challenges that are enhanced by the use of low k materials.